A protein called HSP90 (Heat Shock Protein 90) may have played a role in the loss of eyes in a species of cave-dwelling fish which evolved from surface-dwelling ancestors.
Imagine living in perpetual darkness in an alien world where you have to find food quickly by touch or starve for months at a time. The limestone caverns of Mexico's Sierra del Abra Tanchipa rainforest contain deep cisterns cloaked in utter blackness. This is where researchers at the University of Cincinnati traveled to find a little fish (Astyanax mexicanus) that has evolved to feast or endure famine entombed hundreds of feet below the ground.
Few animals have ignored the warning “use it or lose it” as spectacularly as the Mexican blind cavefish (Astyanax mexicanus), which no longer has eyes. Now scientists may have solved the riddle of why the fish lost their eyes in the dark. With food so scarce in caves, the animals have to save their energy and their systems are under constant stress.
New genetic mutations aren’t the only way for organisms to adapt to changes in their environment. A growing body of evidence suggests that organisms can harbor mutations whose effects are masked until times of stress.
The mediator is a protein called HSP90, which helps proteins fold so that they can function properly. According to Susan Lindquist, a biologist at the Whitehead Institute for Biomedical Research in Cambridge, Mass., this protein under normal conditions helps even mutated proteins fold. But under stressful conditions, more proteins need help from HSP90, and there isn’t enough to go around. The result is that the misfolded proteins can no longer function properly, which triggers a range of strange outcomes, such as square eyes or shriveled wings in fruit flies. “All organisms are near the precipice of a protein-folding crisis,” Lindquist said.
Hsp90 is one of the most abundant proteins expressed in cells accounting for 1–2% of all cellular proteins within non-stressed cells. It acts as a molecular chaperone and when coupled with other co-chaperones, it aids the folding of newly synthesized proteins, enabling them to be folded correctly as well as helping to degrade misfolded proteins. Hsp90 also facilitates the transport of proteins across the membranes of various organelles including the endoplasmic reticulum and prevents aggregation of proteins which have been partially or fully unfolded.
"The traits they've lost are very conspicuous—their eyes, their pigmentation," Gross said. "The beauty of studying cave animals is it's a very robust model for understanding why features are lost, and it's a simple, stable set of environmental pressures that cause those features to go away."
Although the findings do not prove that HSP90-masked variation helped the fish change their eyes, they lend the idea plausibility. Exactly how stress on HSP90 induces variation is still mysterious, but it is a topic of active research.